Conventionally in the sheet carrying information is carried on the layered face, a pressure-sensitive adhesive layer is set on the layered face all over, partially or linearly in the pattern touching oppositely so that the layered sheets adhere each other, usually when layered. In this pressure-sensitive adhesive which is also referred to as self-adhering pressure-sensitive adhesive, each polymer adheres closely through self-diffusion or anchor effect by strong pressure exerted in the state where adhesive layers are touched oppositely each other, and perpetual adhesiveness or detachable adhesiveness is embodied depending on the composition type or degree of pressuring.
As an example of such sheets for carrying information, a postcard system and the like which have concealability and confidentiality have been made practicable becoming popular.
Examples of postcard systems include those where postcards carrying different information such as private business, copied and printed information are folded, cut-folded or where separate post cards are layered.
In these postcard systems, an adhesive layer is bonded by pressure detachably in different modes of layering to hide concealable and confidential information before mailed, then the hidden information is red by the recipient's detaching the layered face.
Further, other examples of postcard systems which have concealability and confidentiality include the one where an adhesive layer is bonded by pressure undetachably to confidential information before mailed, then the hidden information is red by the recipient's tearing off the adhering part and reopening the layered face.
Adhesive composition forms to be applied on the substrate of the sheet for carrying information such as concealable and confidential postcard systems described above include solution, aqueous solution, emulsion (latex), nonsolvent and solid. Adhesive compositions in the form of aqueous solution and emulsion are generally aqueous while those in the form of solution, nonsolvent and solid are generally nonaqueous.
As aqueous pressure-sensitive adhesive compositions, emulsions of natural rubber latex are popular which have advantages including that they have high adhesive strength, different fillers may be added, they can be applied also on the high absorption substrate such as high quality paper. However, natural rubber emulsions are not suitable for pattern application where partial area is applied because viscosity of the emulsion is low, the applied part of the substrate contracts and so on, though overall application can be exerted easily. In addition, drying after application sometimes requires long time because the medium is water.
On the other hand, as nonaqueous pressure-sensitive adhesive compositions, radiation setting medium including acrylic ones are commonly used, for example, acrylic cured material which is cured by radiation such as an ultraviolet ray, electron ray, X-ray, I-ray and g-ray is used as a pressure-sensitive adhesive as described in Japanese Patent Laid-Open No. 10-265742. For such radiation setting pressure-sensitive adhesive compositions, there are advantages including that drying time after application is short, both overall and pattern application can be easily exerted. However, sometimes lacked adhesive strength and sufficient filler could not be added as well as they had sometimes difficulty when applied on high absorption substrate such as high quality paper because they were readily absorbed in the substrate.
Further, when two pressure-sensitive adhesive layers obtained from adhesive compositions described above were bonded by pressure, thereafter, when the pressure-sensitive adhesive layer bonded by pressure was detached, detachment between the substrate and the pressure-sensitive adhesive layer sometimes occurred resulting in damaging the information because adhesive strength between the substrate and the pressure-sensitive adhesive layer was insufficient.